The Ionic liquids are widely used in various applications with ever growing interest. However, for millions of potentially available ionic liquids, the properties of a few percentages of them have been measured. Meanwhile, the majority of the samples to be analyzed in the fields of analytical chemistry and biomedical science are within liquid environment. Therefore, the analysis of liquid must be performed. When measuring the properties of liquid, we usually give attention to its density and viscosity.
In prior art, when measuring the density and viscosity of liquid, an analytical electronic balance and a viscometer are used separately. First, the mass of the liquid is weighed by an analytical electronic balance, and the density of liquid can be obtained according to the formula (1):
                                          ρ            l                    =                                    m              l                                      V              l                                      ;                            (        1        )            
Then, the viscosity of liquid is measured by a viscometer, such as rotary viscometer, ultrasonic viscometer and capillary viscometer. The conventional method mentioned above has certain limitations and deficiencies for analytical chemistry and biomedical field, which are mainly as follows:
(1). The accuracy of currently available analytical electronic balances can only reach the magnitude of microgram, while the higher accuracy is pursued in analytical chemistry and biomedical field;
(2). In analytical chemistry and biomedical field, usually, the price of sample is extremely expensive, while the conventional method requires a relatively large amount of sample;
(3). The conventional method cannot provide the analysis results online and real time.
The method for measuring the properties of liquid based on quartz crystal microbalance sensor can compensate for the mentioned deficiencies to a certain extent. Thereby the application of quartz crystal microbalance sensor has become a research focus in liquid property measurement.
The pressure sensor based on quartz crystal microbalance has been commercialized, and widely applied in biology, chemistry, physics and other fields. However, only few researches relate to the liquid property measuring with quartz crystal microbalance.
So far, only a few papers concerning the method for measuring the density and viscosity of an unknown solution based on QCM sensor have been published, and certain limitations and deficiencies still exist. K. Kanazawa, J. G. Gordon et al. proved that the frequency shift of a QCM sensor and the property of the contacted liquid, i.e. the product of the density and viscosity, exist certain relationship (K. K. Kanazawa, J. G. Gordon, Frequency of a quartz microbalance in contact with liquid, Anal. Chem. 57 (1985) 1770-1771). However, they ignored the fact that the mass of liquid can cause frequency shift of the QCM sensor as well. Ward, M. D and Buttry, D. A et al. published “In situ interfacial mass detection with piezoelectric transducers, Science 1990, 249, 1000-1007” in Science in 1990, they reported that the mass of liquid on the surface of the QCM sensor can also shift the frequency. After that, Schumacher group and A. R. Loveday group verified the validity of the conclusion (R. Schumacher, The Quartz Microbalance: A Novel Approach to the In-Situ Investigation of Interfacial Phenomena at the Solid/Liquid Junction, Chem. Int. Ed. Engl. 1990, 29, 329-343; Hillman, A. R.; Loveday, D. C.; Swam, M. J. J. Transport of neutral species in electroactive polymer films, Chem. Soc. Faraday Trans. 1991, 87, 2047-2053). Later, Martin et al. added liquid mass effect to the frequency shift of the QCM sensor in liquid phase applications. (Stephen J. Martin, Victoria Edwards Granstaff, and Gregory C. Frye, Characterization of a Quartz Crystal Microbalance with Simultaneous Mass and Liquid Loading, Anal. Chem. 1091, 63, 2272-2281).
Martin and coworkers report a continuum model that describes the QCM simultaneously loaded by a thin surface mass layer and a semi infinite Newtonian liquid (“Stephen J. Martin, Victoria Edwards Granstaff, and Gregory C. Frye, Characterization of a Quartz Crystal Microbalance with Simultaneous Mass and Liquid Loading, Anal. Chem. 1091, 63, 2272-2281”), but they think that with only one QCM sensor, the density and viscosity of liquid cannot be extracted. With such assumption, they invented a measurement method with dual QCM sensors and applied for a patent (Patent No. US005201215A) in 1991. In that method, one QCM sensor is with a smooth surface which only responses to the product of liquid density and viscosity, while the other QCM sensor is with textured surface which not only response to the product of liquid density and viscosity but also an additional response to the liquid density. In 2009, N. Doy, G. McHale and M. I. Newton et al. made further research based on the previous work of Martin. They put forward the explicit expression of liquid density and viscosity based on the dual QCM sensors measurement method (“Separate Density and Viscosity Determination of Room Temperature Ionic Liquids using Dual Quartz Crystal Microbalances”).
The two methods mentioned above all adopt two different QCM sensors, thus leading to a more complex process and less accurate results.
In 2011, Atsushi Itoh and Motoko Ichihashi et al. simultaneously measured the frequency shift and the conductance shift of QCM sensor, and established equations to obtain the density and viscosity of liquid (Atsushi Itoh and Motoko Ichihashi, Separate measurement of the density and viscosity of a liquid using a quartz crystal microbalance based on admittance analysis (QCM-A), Meas. Sci. Technol. 22(2011)015402). Although only single QCM sensor is used, the measurement process is more complex and the accuracy of the result needs to be improved, and moreover, they did not gave the explicit expression of liquid density and viscosity.